In Streptomyces griseus, A-factor (2-isocapryloyl-3R-hydroxymethyl--butyrolactone) serves as a microbial hormone that switches on many genes required for streptomycin production and morphological development. An open reading frame (Orf1) showing high sequence similarity to oligoribonucleases of various origins is present just downstream of adpA, one of the A-factor-dependent genes. Orf1 was named OrnA (oligoribonuclease A) because it showed 3'-to-5' exo-oligoribonuclease activity, releasing [³²P]CMP from ApCpC[³²P]pC used as a substrate. Reverse transcription-PCR and S1 nuclease mapping analyses revealed that ornA was transcribed from two promoters; one was a developmentally regulated, A-factor-dependent promoter in front of adpA, and the other was a constitutive promoter in front of the ornA coding sequence. Transcription of ornA was thus additively enhanced at the initiation stage for secondary metabolism and aerial mycelium formation. ornA-disrupted strains grew slowly and scarcely formed aerial mycelium. ornA homologues were distributed in a wide variety of Streptomyces species, including S. coelicolor A3(2), as determined by Southern hybridization analysis. Disruption of the ornA homologue in S. coelicolor A3(2) also caused phenotypes similar to those of the S. griseus ornA strains. The OrnA oligoribonucleases in Streptomyces species are therefore not essential but play an important role in vegetative growth and in the initiation of differentiation.